Leak testing apparatus

ABSTRACT

An apparatus for testing for leaks in a test item by subjecting the item to a test pressure, closing a valve to contain the pressure in the test item, and measuring the change in the pressure in which the apparatus measures the initial pressure in the test item after the item is closed and compares the change in pressure relative to the initial pressure value thereby determining the extent of any leak in the test item. The test item may be subjected to a pressurized gas or vacuum. In subjecting the test item to a positive pressurized gas, an electrical signal is generated providing a compensating exponential signal of equal and opposite value to the dissipation of the adiabatic heat as the pressurized gas in the test item cools after closing.

United States Patent 1 1 Delatorre et al.

LEAK TESTING APPARATUS Inventors: Leroy C. Delatorre, Spring; William J.Rapson, Jr.; Paul H. Lemson, both of Houston, all of Tex.

Assignee: Uson Corporation, Houston, Tex.

Filed: Apr. 24, 1972 Appl. No.: 246,633

US. Cl. 73/49.2 Int. Cl. GOlm 3/02 Field of Search 73/492, 49.3, 52, 40

References Cited UNITED STATES PATENTS 4/1969 Bossert, Jr. 73/493 X4/1972 Mills 73/492 X 3/1957 Lawrance et al. 2/1959 Mamzic 73/40 FOREIGNPATENTS OR APPLICATIONS 2/1941 Great Britain ..73/49.-3

[451 Apr. 2,1974

Primary Examiner Richard C. Queisser Assistant Examirier,loseph W.Roskos Attorney, Agent, or FirmJefferson D. Giller; James F. Weller;William A. Stout [57] ABSTRACT An vapparatus for testing for leaks in atest item by subjecting the item to a test pressure, closing a valve tocontain the'pressure in the test item, and measuring the change in thepressure in which the apparatus measures the initial pressure in thetest item after the item is closed and compares the change in pressurerelative to the'initial pressure value thereby determining the extent ofany leak in the test item. The test item may be subjected to apressurized gas or vacuum. ln subjecting the test item to a positivepressurized gas, an electrical signal is generated providing a comipensating exponential signal of equal and opposite value to thedissipation of the adiabatic heat, as the pressurized gas in the testitem cools after closing.

4 Claims, 4 Drawing Figures TYPICAL MECHANICAL INSTALLATION PRESSURETRANSDUCER TES g AIR guPPLY ITEM I2 SOLENOID PRESSURE VACUUM SOURCEVALVE REGULATOR PAIENTED W 2 SHEET 1 BF 2 TYPICAL MECHANICALINSTALLATION FIG.

PRESSURE TRA NSDUCER fiAIR SUPPLY 0R VACUUM SOURCE TES , ITEM Has ""1'"'""""'-v'- l I I I l l I l I 74 r--- -v-r-- l I f l 77 I I 1 I i FIG. 4

NEWS R E WW W C W E 0 E T T w w v? T A B C F- E E M M M T T T E m u HSnO A OI E PT\N H M O O- .I 0 CIC T T A A S I E Ml W DB AA K 0N R T AUOvM EU mw L TI 2 WW I w mm m VF TIME

LEAK TESTING APPARATUS BACKGROUND OF THE INVENTION Pressure decay leaktesting is a way of determining leakage in a test item by pressurizingthe test item to a given test pressure, closing off the pressure sourceand monitoring the decay in pressure which is a function of leakage.This has been done in the past by measuring the pressure decay from thetest item or by using a reference test item identical to or simulationof the test item with a differential pressure transducer therebetween.While the concept of measuring a leak in an item by monitoring pressuredecay is simple, there are a number of difficulties that are encounteredin actual practice. The most important of these are: (l) adiabaticheatingof the pressurizing gas occurs when the test item is pressurizeddue to compression. This results in a pressure decay after the test itemis closed due to the-dissipation of this adiabatic heating therebycausing an error in the pressure decay reading. (2) Leak sensitivityvaries inversely with test volume and also directly with decay time andpressure change sensitivity. (3) Any temperature change of the test itemduring the decay measuring period can invalidate the test results due tothe dependence of pressure on temperature. (4) Any change in theinternal volume of the test item due to pressure effects will also causea pressure decrease.

In general, it is desirable to be able to test as fast as possible inorder to effect a maximum throughput rate. However, in using short testcycles, there is the problem of adiabatic heating and cooling. That is,when the test item is pressurized, there will be adiabatic heating ofthe pressurizing gas due to compression. After the test item isclosedoff, the pressurized gas will cool dissipating the adiabatic heat.During a long test cycle, enough time can be allocated for stabilizationto allow any adiabatic heat to dissipate before making the measurement.But in the case of short test cycles, this stabilization time can exceedthe total test time. One feature of the present invention is to providea compensation signal to reduce the effect of the dissipation of theadiabatic heat. The present invention compensates by electricallygenerating a signal of proper magnitude and duration and introducing itinto the signal output of the pressure measuring transducer to cancelthe exponential component due to the adiabatic effect.

As previously indicated, one standard approach for testing an item forleaks is to use a reference vessel identical to the test vessel togetherwith a differential pressure transducer so that certain environmentaleffects are cancelledaThe use of the reference vessel technique hascertain limitations such as: (l) A ruptured or'test vessel having alarge leak resultsin the full test pressure being applied to thedifferential transducer with possible damage. (2) Some test items suchas those made of plastic cannot take continuous repeated pressurecycles. (3) The reference vessel can become cooled by the adiabatic heatduring continuous fast testing thereby reducing adiabatic heatcancellation. This effect occurs because the adiabatic heat ofpressurization is largely dissipated in the test cycle so that ondepressurization at the end of test a reverse adiabatic cooling occurs.When a new test item is quickly tested the cooling is still present inthe reference vessel which results in partial cancellation of theadiabatic 2, heat onpressurization. This, of course, does not occur inthe test vessel since it is replaced with each test.

The present invention is directed to an improvement by providing atemperature compensation circuit to reduce the effects of adiabatic heatdissipation and also creep in plastic items and allows the use of asimple nondifferential-type pressure measuringtransducer.

Furthermore, in the reference vessel type testing method, difficultyarises in trying to quickly pressurize both the test and referencevessel to substantially the same pressure so that the test item isreferenced to a known starting point; The present invention provides amemory circuit which memorizes whatever pressure level was actuallyreached when the vessel was pressurized and then looks at deviation ofpressure from this point whichis its own artificial zero reference pointand overcomes the requirement for accurately press'urizing the testvessel to a predetermined value. In addition, the memory circuit has theadvantage of allowing a gross leak to be detected.

The present invention can also be applied to testing with a referencevessel All of the advantages of testing with an artificial zero stillapply and the adiabatic heat compensation can be used to take up anymismatch between the test item andthe reference vessel.

SUMMARY The present invention is directed to an apparatus for testingleaks in a test item in which the item is subjected to a pressurized gasand closed, and a transducer measures the pressure in the closed item.An electrical temperature compensating circuit is connected to thetransducer for providing an electrical signal equal and opposite to theeffects of the dissipation of adiabatic heat of the pressurized gas inthe test item and creep in the test item.

Yet a still further object of the present invention is to provide anapparatus for testing for leaks in a test item which is subjected to apressurized gas and closed in which the change in the pressure of thegas after the test item is closed is measured in which means areprovided for measuring and memorizing the pressure value after-the testitem is closed Means are provided for comparing the change in thepressure with the memorized pressure value thereby measuring the extentof any leak in the test item,

Yet a still further object of thepresent invention is an apparatus fortesting leaks in a test item which is subjected to the pressurized gasand closed and a pressure measuring transducer measures the change inthe pressure of the gas in the test item in which a first amplifier isprovided connected to the transducer for continuously measuring theoutput from the transducer. A feedback loop is connected across thefirst amplifier including a second amplifier having a memory retainingmeans, and means for actuating the feedback loop when the test item isclosed, to provide a reference point to the first amplifier so that theoutput from the first amplifier relative to the reference point is ameasurement of the extent of leak in the test item.

Still a further object of the present invention is the provision of anapparatus for testing leaks in a test item in which the effects ofambient temperature changes and adiabatic temperature changes arereduced by subjecting the test item to a negative pressure or vacuum,and closing the test item while being subjected to a vacuum, andproviding transducer means for measuring the pressure change in theclosed item to determine the extent of any leak in the test item.

Other and further objects, features and advantages will be apparent fromthe following description of a presently preferred embodiment of theinvention, given for the purpose of disclosure, and taken in conjunctionwith the accompanying drawings where like character references designatelike parts throughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic ofthe mechanicalinstallation of the present invention,

FIG. 2 is an electrical block diagram of the present invention,

FIG. 3 is an electrical schematic of the compensation circuit of thepresent invention, and

FIG. 4 is a graph illustrating offsetting of the dissipation of theadiabatic heating with the compensation network.

DESCRIPTION OF THE PREFERRED I I EMBODIMENT While the present inventionwill be described, for purposes of illustration only, in connection witha type .of pressure measuring leak detector system having anondifferential measuring transducer, it is understood that the presentinvention may also be used with other types such as the use of areference vessel and differential transducer type pressure measuringsystem.

Referring now to the drawings, particularly to FIG. 1, the test item tobe tested is connected to a supply line 12 and is subjected to apressurized gas from a suitable air supply 14, or a vacuum source aswill be more fully described hereinafter, through a pressure regulator16 and a normally-closed solenoid valve 18. The solenoid valve 18 isopened to allow the test item 10 to be subjected to a pressurized gasafter which the valve 18 is closed. A conventional pressure transducer20 measures any pressure decay in the test item 10, and, of course, thepressure change in the test item 10 is an indication and measurement ofthe extent of a leak, if any, in the test item 10. Any pressure changefrom the initial pressure after closure for a predetermined timeinterval is measured to determine possible leakage.

Referring to FIG. 2, the pressure transducer 20 continuously measuresthe trapped pressure in the test item 10 and its output is transmittedto an amplifier 22. The output from the amplifier 22 is transmitted to ago-nogo logic circuit 24 which determines whether the output from theamplifier 22 is above or below a predetermined amount which isdetermined by adjusting a set point 26. The logic circuit 24, after thepredetermined test interval, measures the amount of pressure change inthe test item 10 by means of the transducer 20 and amplifier 22 and ifthe amount of pressure change is below the set point, signal light 28lights up indicating that the test specimen is acceptable. If thepressure change which is measured by the logic circuit 24 is above theset point, the circuit 24 actuates light 30 indicating that the testitem 10 has an unacceptable leak and is therefore rejected.

Referring to FIG. 1, the test is made by connecting the test item 10 tothe line 12. Referring to FIGS. 1 and 2, the test is initiated bydepressing start button 32, either manually or automatically, foractuating a solenoid timer 36, which in turn actuates a solenoid relay38 for opening the normally closed solenoid valve 18 to allow the airsupply 14 to pass through the pressure regulator 16 and solenoid valve18 to pressurize the test item 10. At the same time, the inhibit circuitthrough line 40 inhibits actuation of the indicator lights 28 and 30.Also at this time period, the solenoid timer 36 through a line 42actuates a compensation network 44 which will be more fully describedhereinafter.

After a predetermined length of time sufficient to pressurize the testitem 10, the solenoid timer 36 deactuates the solenoid relay 38 which inturn closes the solenoid valve 18 thereby trapping the pressurized airin the test item 10. If desired, a delayed timer 46 may be provided todelay the start of the test measurement to allow for pressurestabilization in the test item 10; however, timer 46 may be set to zeroor deleted to provide a minimum test cycle time.

The pressure transducer 20 is continuously measuring the trappedpressure in the test item 10 and transmitting the measurement to theamplifier 22. The delay timer 46 actuates through line a zeroing andmemory circuit generally indicated by the reference numeral 50 which isa feedback loop connected to across the amplifier 22. Circuit 50generally includes a zeroing circuit 52, a second amplifier 54 and amemoryretaining means such as capacitor 56. The zeroing circuit andmemory circuit 50 is used to establish an artificial zero referenceagainst which pressure deviation in the test item 10 may be measured.That is, circuit 50, after pressure is trapped in the test item 10,receives an initial measurement of the pressure in the test item 10 fromthe output of the amplifier 22 and holds this initial measurement levelin the capacitor memory 56 thereby nulling the output from the amplifier22 to zero. The circuit 50 electrically memorizes the pressure input atthe beginning of the testing cycle and then allows the pressuretransducer 20 and amplifier 22 to look for a pressure decay from thisartificial zero reference point during the testing interval time. Thememory circuit 50 provides an important feature in hat it memorizeswhatever pressure level was actually reached in the test item 10 andthen looks at pressure deviation from this point which is the artificialzero reference point from which the set point 26 is measured. Therefore,the present zeroing and memory circuit overcomes the problem encounteredin a reference vessel testing apparatus in which the test item andreference item must be quickly pressurized to substantially the samepressure. Other wise the zero point is inaccurate with reference to theset point which is fixed.

When the zero circuit 52 has reached zero, it actuates the test timer 58by means of zero confirm signal line 47 which enables the accept-logiccircuit 24. The input to the amplifier 22 is compared with the initialpressure which is now stored in the memory capacitor 56 and the pressuredecline from this initial pressure is transmitted by the amplifier 22 tothe logic circuit 24 for the test period set by the test timer 58 todetermine possible leakage of the test item 10. If the output from theamplifier 22 exceeds the set point 26 at any time during the test cycle,the logic circuit 24 stores the reject condition and displays it onreject light 30 at the end of the test. If the output of the amplifier22 is less than the set point 26, the logic circuit 24 actuates theaccept indicating light 28 at the end of the test.

Of course, if the test item 10 has a gross leak, that is a leak so largethat all of the pressure is depleted before the test begins, no pressuredecay is possible during the test time and therefore the accept lightwould erroneously indicate that the test item is satisfactory. However,since the zeroing and memorizing circuit 50 measures and memorizes thepressure level in the test item after the solenoid valve 18 is closed,indicating means such as a pressure meter having an output such as ameter 70 is connected to the circuit 50. Meter 70 may measure percentpressure, normally the full scale pressure of the transducer. Therefore,if the meter 70 indicates that the initial pressure measured andmemorized by the circuit 50 after the solenoid valve 18 closes is zero,this is an indication that the test item 10 has not been pressurized orhas a gross leak, and the test is invalid.

An additional meter having an output meter scale 72 may be providedconnected to the output of the amplifier 22 such as a pressure changemeter which scale is normally calibrated a percentage of the full scaleof the pressure meter 70. Thus meter 72 provides a visual observation ofthe change in pressure of the test item 10 inaddition to the indicatinglights 28 and 30.

As previously mentioned, when the test item 10 is .pressurized with gas,the pressurized gas is adiabatically heated due to compression.Therefore, after thesolenoid valve 18 is closed, the pressurized gas inthe item 10 will cool dissipating the adiabatic heat. One feature of thepresent invention is the feature of negatively pressurizing the item 10or creating a vacuum in the test item 10 instead of positivelypressurizing the test item 10. By placinga vacuum in the test item 10,there will be no adiabatic heating and cooling, and thus the temperatureerror due to adiabatic heating and cooling is overcome as well astemperature changes caused by changes in ambient temperature. Therefore,the pressure source 14 of FIG. 1 may be either a positive air supply ora vacuum source. In the case of vacuum testing, a leak in the test item10 will allow the pressure in the test item 10 to increase which ismeasured by the pressure transducer 20. It is noted that the meter 72has a zero point in the middle of the readout meter. Movement to theleft is a pressure loss measurement for a positive pressurizing of thetest item 10. Movement to the right is an indication of a pressureincrease which would occur to indicate leakage in a vacuum measuringsystem.

However, it may be desirable to use a positive pressure measuringsystem, and in that event the pressure invention is provided with acompensating network 44 to compensate for the pressure loss caused bycooling of adiabatic heating through the test item 10 back to ambienttemperature as well as compensating for elastic creep which sometimesoccurs in test items of plastic or rubber. The adiabatic heat cooling isessentially an exponential pressure loss relative'to time. In order tocancel out the exponential decrease of transducer output voltage, thecompensating network 44 introduces an exponential increase in thetransducer 20 excitation voltage which is reflected in the transduceoutput voltage as an increase.

Referring now to FIGS. 3 and 4, the schematic of the temperaturecompensating circuit 44 and its results are best seen. A conventionalvoltage regulator 74 is provided receiving a suitable nonregulatedvoltage 76 and providing a regulated voltage output. The compensationnetwork 44 may include a capacitor 77, a first resistor 78, a secondresistor 80, and a switch 82. When the solenoid valve 18 is opened attime A to allow the test item 10 to be pressurized, the switch 82 issimultaneously moved to contact 84 allowing the capacitor 77 to bedischarged through the resistor 78. During this time, the cooling of thetest gas begins and follows the substantially exponential curve 100 inFIG. 4. At time B, the solenoid valve 18 is closed, and at the same timethe switch 82 is moved from contact 84 to contact 86. The capacitor 77is now allowed to charge through resistor 80. The current I through thecompensating network 44 decreases from a high value at the outset toessentially zero after five time constants where:

and T is the time constant in seconds if R is the resistance of resistorin ohms and C is the capacitance of capacitor 77 in farads. The value of1 during the time the switch 82 is on contact 86 is:

The output from the compensation circuit 44 is connected to a bridgecircuit type strain gauge transducer 90 and thus is reflected in theoutput of transducer 20.

The voltage output of the compensating circuit 44 versus time isindicated in FIG.-4 as 102. It is noted from the graph in FIG. 4 thatfrom time B, when the so lenoid valve 18 is closed, to time C, the timewhen the test is over, that the compensating network output 102 cancelsout the cooling due to the adiabatic heating as shown in to produce acomposite voltage 104. The composite voltage 104 obtained by adding thecurves 100 and 102 is substantially constant. The magnitude of thecompensation as well as the slope of curve 102 is changed by varying thevariable resistor 80 and the capacitor 77. The magnitude and timeconstant are larger for larger test items. In comparing equal volumedtest items constructed of brass and plastic, the time constant is muchlonger for the plastic item, while the magnitudes are about the same forboth cases.

Thus, the present apparatus tests an item 10 for leakage by charging itwith a-gas, preferably air, to the test pressure through the solenoidvalve 18. When the valve 18 is closed, the pressure transducer 20continuously measures the trapped pressure in the test item 10. Theinitial pressure measurement after closure of the valve 18 is stored inthe memorizing circuit 50 and any pressure decay from the initialpressure is determined after a time period to measure possible leakage.Precise pressure regulation is not required since the electronic memorycircuit 50 resets at an artificial zero point on every test. Inaddition, vacuum testing of the test item 10 may be provided if desiredto overcome temperature problems. And if positive pressure testing isdesired, a compensating circuit 44 may be used to cancel out the effectsof cooling of the adiabatic heat and also compensate for creep in testitem 10.

The present invention, therefore, is well adapted to carry out theobjects and attain the ends and advantages mentioned as well as othersinherent therein. While a presently preferred embodiment of theinvention is given for the purpose of disclosure, numerous changes inthe details of construction and arrangement of parts may be made whichwill readily suggest themselves to those skilled in the art and whichare encompassed within the spirit of the invention and the scope of theappended claims.

What is claimed is:

1. An apparatus for testing leaks in a test item comprising,

means for subjecting the item to a pressurized fluid,

means for closing the test item while being subjected to a pressurizedfluid,

transducer means for measuring the pressure in the closed item,

electrical temperature compensating means connected to the transducerproviding an electrical signal equal and opposite to the effects of thedissipation of the adiabatic heat of the pressurized fluid in the testitem on the transducer output,

means connected to the transducer for measuring an initial pressurevalue in the item after the test item is closed, and

means for comparing the change in the pressure relative to the measuredinitial pressure value thereby determining the extent of any leak in thetest item.

2. The apparatus of claim 1 wherein the compensating means includes,

a condenser and a resistor,

switching means connecting the condenser and resistor to a voltagesource whenthe test item is closed thereby providing a substantiallyexponential output.

3. The apparatus of claim 1 including,

a first amplifier connected to the transdurcer for continuouslymeasuring the output from the transducer,

a feedback loop connected across the first amplifier including a secondamplifier having memory retaining means,

means for actuating the feedback loop when the test item is closed toprovide a reference point to the first amplifier whereby the output fromthe first amplifier relative to the reference point is a measurement ofthe extent of any leak in the test item.

ciently pressurized.

1. An apparatus for testing leaks in a test item comprising, means forsubjecting the item to a pressurized fluid, means for closing the testitem while being subjected to a pressurized fluid, transducer means formeasuring the pressure in the closed item, electrical temperaturecompensating means connected to the transducer providing an electricalsignal equal and opposite to the effects of the dissipation of theadiabatic heat of the pressurized fluid in the test item on thetransducer output, means connected to the transducer for measuring aninitial pressure value in the item after the test item is closed, andmeans for comparing the change in the pressure relative to the measuredinitial pressure value thereby determining the extent of any leak in thetest item.
 2. The apparatus of claim 1 wherein the compensating meansincludes, a condenser and a resistor, switching means connecting thecondenser and resistor to a voltage source when the test item is closedthereby providing a substantially exponential output.
 3. The apparatusof claim 1 including, a first amplifier connected to the transdurcer forcontinuously measuring the output from the transducer, a feedback loopconnected across the first amplifier including a second amplifier havingmemory retaining means, means for actuating the feedback loop when thetest item is closed to provide a reference point to the first amplifierwhereby the output from the first amplifier relative to the referencepoint is a measurement of the extent of any leak in the test item. 4.The apparatus of claim 3 including, indicating means connected to theoutput of the feedback loop for indicating if the tEst item issufficiently pressurized.